In recent years, a number of techniques for depositing, growing, and processing thin films of various materials with optimum control of purity, composition and microstructure have become of extreme interest and the subject of intensive research by workers in the field. Such techniques include thermal evaporation, DC sputtering, RF sputtering, ion beam deposition (IBD), ion cluster beam deposition (ICBD), chemical vapor deposition (CVD) (and related techniques, such as PECVD, MOCVD, etc.), electro-plating, molecular beam epitaxy (MBE), liquid phase epitaxy (LPE) deposition and laser assisted and/or enhanced deposition. The latter may include laser thermochemical deposition and laser photochemical deposition. Each of the techniques for forming thin films carry with them certain advantages and disadvantages.
The structure of thin films can be amorphous, polycrystalline, epitaxial or polycrystalline with a preferred orientation. In amorphous films, the atoms are not arranged in any crystalline order. In polycrystalline films, there are many small regions, called grains, in which the atoms are arranged in a regular crystalline order, but the grains have a random crystallographic orientation with respect to each other. A single crystal film is a special form of thin film growth, in which all atoms of the film belong to one single large grain. In other words, the atoms are crystallographically ordered within one single lattice structure. The term "epitaxial", as applied to polycrystalline or single crystal films, indicates that the lattice orientation of the film is aligned with the underlying crystalline substrate in such a way that it forms a commensurate, discommensurate, or uncommensurate interface. A polycrystalline film with preferred orientation, is composed of many small grains, in each of which the atoms are arranged in a regular crystalline order, and one or more of the crystalline axes of the majority of said regions are close to being parallel.
A thin film can be the same material (that is, the same element or compound) as the substrate, or it can differ in chemical composition from the substrate. If the film is epitaxial, the former is called "homoepitaxy" and the latter "heteroepitaxy".
The present invention pertains to both MBE and IBD, therefore, the pros and cons of each are discussed below.
MBE is basically an ultra-high vacuum evaporation technique in which effusion cells, sometimes referred to as Knudsen cells, containing sources of the material to be deposited are introduced into an ultra-high vacuum chamber, along with a substrate. The sources are heated until vaporized. Because of the high vacuum, the source atoms or molecules in the vapor move with few gas phase collisions and deposit on any surface they reach. When they land on a hot substrate surface, they are physisorbed and sometimes chemisorbed, forming a high quality (i.e., low defect density) epitaxial film. Low defect density means that the number of defects, such as, dislocations, twin boundaries, stacking faults, or other crystalline imperfections is relatively low per unit volume.
There are two main advantages to using MBE for growth of thin films.
First, the ultra-high vacuum provides a clean environment for deposition, and second, a number of source cells can be provided in the chamber with different source materials. By using a mechanical shutter between each source and the substrate, successive or simultaneous beams of molecules or atoms from successive sources can be directed at the substrate and very abrupt transitions can be obtained between multi-layered films of different compositions (heterostructures) and/or doping levels.
The main disadvantages associated with the MBE system are the high cost of the reactor coupled with the dedicated nature of each system. Because the source elements are deposited in a gas phase, the entire chamber becomes contaminated with source material. Hence, each chamber can only be used for a limited number of reactions compatible with the contaminating material. For example, if an arsenic effusion cell is introduced in a Si MBE chamber, the whole reactor will be contaminated with arsenic, once this cell has been used, to the extent that the chamber would be rendered useless for growth of materials whose properties are altered by very small amounts of arsenic. An example of such a material is NiSI.sub.2, grown to form a low-resistivity ohmic contact with As-doped Si. Arsenic contamination degrades the properties of the contact. Another example is boron doped Si. The p-type boron doped Si material cannot be grown in an arsenic contaminated environment, because the arsenic converts the Si material to n-type. Decontamination of a reactor is extremely impractical, as it requires a complete dismantling and cleaning of the system.
As a consequence of this problem for MBE facilities, each material that may cross-contaminate with another material used in that heterostructure has to be grown in a separate chamber in order to grow heterostructure semiconductors. This leads to systems with multiple chambers, i.e., up to 8, with each chamber valued between $600,000 and $1,000,000.
Another disadvantage of MBE is the inherent limitation of the choice of dopant. If the melting or sublimation temperature of a dopant is too high, then the confinement of a solid source in a Knudsen cell is impractical, because of the high temperatures required. For example, Boron, whose melting point is very high (2300.degree. K.), cannot be used as a dopant in present MBE technology. Alternate p-type dopants (A1,Ga) must be used despite their inferior diffusion and segregation properties, as compared to Boron.
Another disadvantage of MBE is the difficulty of accurately controlling dopant profiles. This difficulty is due both to the angular variation of evaporant flux and to the dependence of sticking coefficient on temperature.
In addition, the MBE process requires heating the sorce material at temperatures above the melting point of that material, in order to form a vapor. When refractory materials are used, such as silicon or transition metals, such high temperatures pose problems. For example, molten silicon is a universal solvent. At present no reliable material has been found that can contain molten silicon in order to form an MBE source of silicon of the Knudsen cell type.
Electron beam evaporators therefore have to be used instead of an effusion cell. However, this is a difficult and expensive technology, adding to the cost of the several reactors necessary to eliminate cross-contamination. Also, during deposition conducted at such high temperatures, the substrate and the deposition materials can decompose after deposition during the growth of the subsequent film. This makes it impossible, for instance, to grow epitaxial silicon or gallium arsenide with adequate interface quality. It is also impossible to form a silicon based modulation doped superlattice with an atomically sharp doping profile using MBE. This is because undesired diffusion of dopants from one layer to another occurs during subsequent growth of epitaxial silicon. For example, A1 and Ga, the most commonly used p-type dopants used in silicon MBE, are too mobile to maintain sharp dopant profiles, even at the lowest possible temperature for silicon MBE.
Furthermore, when depositing refractory materials in addition to the necessity of using sophisticated electron beam evaporation guns to vaporize the materials, the heat released by the process is two to three times larger than when effusion cells are used. This leads to increased difficulties on all aspects of cooling, shielding, cryoshielding and vacuum technology.
Lastly, the deposition rate of MBE processes is slow (about 1 micron/hour) causing low throughput and the necessity of expensive vacuum technology to reduce gas adsorption during growth to less than a few parts per billion.